Transport of ozone by turbulence and clouds in an urban boundary layer

Abstract

The turbulent fluxes of ozone and latent and sensible heat are computed from fast‐response measurements made aboard a National Oceanic and Atmospheric Administration aircraft over downtown Philadelphia and the surrounding suburbs during the afternoon and evening of August 22, 1979. In the afternoon the ozone flux at a height of 200 m is downward throughout the region with the largest magnitude (−2 (ppb) m s−1) occurring over the urban center. During the afternoon at both 200 m and a few hundred meters below cloud base, the horizontal profile of mean ozone concentration peaks at 130 ppb over the urban core with values of the order of 90 ppb to the southeast and northwest. The urban ozone concentration at 200 m decreases to 35 ppb by early evening. The normalized variances and spectra of vertical velocity, temperature, and ozone show little change with height or location in the urban center and northwest suburbs during the afternoon in good agreement with normalized statistics obtained over rural terrain (Kaimal et al., 1976; Lenschow et al., 1980). Data from a cloud penetration by the aircraft is used to estimate a mean updraft velocity of 4 m s−1 and an updraft area of approximately 1 km2. The flux of ozone due to the mean motion in the updraft is 2 orders of magnitude larger than the turbulent eddy fluxes within the cloud.

Transport of ozone between boundary layer and cloud layer by cumulus clouds

“…However, the larger proportionality constant is due to the 30% contribution of cloud turbulence to the total cloud flux. Greenhut et al [1984]. In that analysis the cloud was divided into three regions, a central updraft surrounded by downdrafts on either side, using the vertical velocity time series.…”

Vertical ozone fluxes and related deposition parameters over agricultural and forested landscapes

“…Lenschow et al (1981) also reported a large negative (~0~) above a highway that was about twice the magnitude of values over nearby agricultural land in Colorado. Greenhut et al (1984) found that (WOJ values were significantly stronger in an urban area than over the surrounding countryside. Ozone flux can be greatly influenced by rapid in-air chemical reactions between ozone and other precursor species, particularly NO,.…”

“…Flights over the Gulf of Mexico have revealed considerably smaller ozone fluxes over water than to pine forests (Lenschow et al, 1982). The results in Greenhut et al (1984) displayed considerable variability in the vertical ozone flux across a large urban region.…”

“…The computer code simulating this high-pass filter function was applied with Tl = 36 s and T2 = 54 s. These values produced a rapid roll-off with a 3 dB point at 0.0093 Hz, which caused a 50% reduction in a 7 km wavelength (Greenhut et al, 1984). These T values were employed after considerable testing with several pairs indicated stable results in the computed variances and fluxes at this scale; no noticeable impact was evident in the region of the peak and at higher frequencies in w spectra or (woa) cospectral results.…”

Ozone production efficiency and loss of NOx in power plant plumes: Photochemical model and interpretation of measurements in Tennessee

“…Fair-weather cumulus clouds have been found to export air out of the boundary layer and inject boundary layer air at higher elevations [e.g., Greenhut et al, 1984;Stull, 1985…”